It is per se known to connect, in the connecting portion between an electronic device which is used as a communication device, such as a telephone set, a facsimile, a modem or the like, and a telecommunication line or a power line, an antenna, or a CRT monitor drive circuit or the like, a surge absorber for protecting electronic components within the device or a printed circuit board to which such components are mounted against destruction due to thermal damage or fire or the like caused by abnormal voltage being applied to portions of the device which can easily suffer electric shock by an abnormal voltage (surge voltage) or abnormal current (surge current) such as lightning surge or electrostatic or the like.
In the prior art, as for example disclosed in Japanese Patent Application, First Publication No. Hei 9-171881, there has been proposed a surge absorber of the discharge type, comprising: element housed within a glass tube, and provided with terminal electrodes at both its ends; a pair of Dumet wires which are inserted into the two ends of the glass tube, and each of which is connected to one of the terminal electrodes, each of them having its ends connected to a lead wire for connection to an external circuit; and cylindrical tube shaped spacers which, along with each surrounding and holding the Dumet wires, are inserted into both the end portions of the glass tube, and seal both end portions of the glass tube. In this case, fluctuations in DC spark over voltage because of the contact between the Dumet wires and the terminal electrodes becoming unstable can easily occur. Furthermore, this surge protector is unreasonable from the point of view of cost, since the cast of materials increase for the larger terminal electrodes.
Furthermore, the electronic devices become more compact, the surface mounted discharge type surge absorber become more popular. A surface mounted surge absorber (of the Murph type) is equipped with terminal electrodes which have no lead wires, and when being mounted upon a substrate, the terminal electrodes are connected to the substrate by soldering. In this type of surge absorber, as for example disclosed in Japanese Patent Application, First Publication Nos. 2002-110311 and 2002-134247, has a surge absorption element with a micro gap. An example of the structure of this type of surge absorber is shown in FIG. 10.
A surge absorption element 1 consists of a ceramic part (insulating part) 3 of circular cylindrical form, upon the circumferential surface of which there is spread a conductive layer 2, with a so called micro gap M being formed at the central portion of this conductive layer 2, and with a pair of cap electrodes being fitted to both ends of this ceramic part 3. This surge absorption element 1 is housed within a glass tube 5 which is filled with seal gases G, and the two ends of this glass tube 5 are sealed by heating at a high temperature by a pair of terminal electrodes 6, thus constituting the discharge type surge absorber.
In recent years, the demand has become more strident for provision of a lower cost surge absorber which, in addition to providing stabilized performance and high quality, is also endowed with durability and high surge resistance capability. Consequently, there have arisen problems with relation to dimensional accuracy of the surge absorption element and the glass tube and the terminal electrodes, and, in particular, a crucial technical assignment has arisen with regard to preventing the occurrence of gaps between the surge absorption element and the enclosing electrodes, and with regard to maintaining secure and reliable contact between the surge absorption.
Furthermore, in recent years, with regard to surge absorbers, a sufficiently responsive performance has been demanded even for applications which require a high surge current capability, as when connecting a telecommunication line or a power supply line or the like. Furthermore, with a Murph type surge absorber, there is a possibility of breaking the glass tube during surface mounting. Due to this, it has been considered to replace the glass tube with a ceramic tube. With a surge absorber which uses a glass tube, the ceramic part is inserted into the glass tube, and after the terminal electrodes have been placed at both ends of the glass tube, in that state, the glass tube is melted in a high temperature oven, and the terminal electrodes are tightly fixed to glass tube so that thereby the glass tube is sealed. When the glass tube is cooled after seal process a sufficiently good ohmic contact is obtained between the terminal electrodes and the conductive layer of the ceramic part, since a residual stress force is set up in the compression direction owing to the thermal expansion coefficient differences between glass tube and ceramic part.
However, when a ceramic tube substitutes for the glass tube, since the thermal expansion coefficients differences of the ceramic tube and the ceramic member is comparatively small as compared with the situation described above, the residual stress which is generated during cooling process is small, so that it may occur that insufficiently good ohmic contact is provided between the terminal electrodes and the conductive layer of the ceramic part. In such a case, the electrical properties of the surge absorber, such as DC spark over voltage, become unstable.
The present invention has been conceived in the light of the above circumstances, and its objective is to provide a lower cost surge absorber which is endowed with excellent durability and a high surge current capacity, and which exhibits stable performance and high quality.